Method of supplying cement slurry and method of drilling or cementing of well using the same

ABSTRACT

Cement slurry is prevented from coming into contact with hardening accelerator in a cementing tool in a simple manner. The method of supplying cement slurry has the steps of: adding capsule C to cement slurry, wherein capsule C is filled with hardening accelerator for the cement slurry; and supplying the cement slurry to a hole, wherein capsule C is added to the cement slurry, and the cement slurry is supplied through cementing tool 105 that is provided in the hole. Capsule C melts or collapses after a specific amount of time passes, wherein the specific amount of time is longer than a flow out time, which is a time for capsule C to flow out of cementing tool 105 after capsule C is added to the cement slurry.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a method of supplying cement slurry, as well as a method of drilling a hole and a method of cementing using the same, and particularly to a method of preventing lost circulation during the drilling of a hole.

Background Art

Drilling mud is used for the drilling of a well for oil, natural gas, geothermal energy and hot springs or of a vertical hole, such as a hot water pipe. The drilling mud is produced by mixing clay, such as bentonite, with water, and is supplied into a vertical hole in order to transport cuttings to the ground or to protect the wall of the hole. Accordingly, loss of the drilling mud from the vertical hole will cause the cuttings to remain in the hole or cause the wall of the hole to collapse, and increases the drilling cost. The phenomenon in which drilling mud flows out of a vertical hole through a specific part of the wall of the hole and through cavities, cracks, permeable layers and the like in the formation (hereinafter, these are referred to as a lost circulation layer) is generally called lost circulation or mud loss. Conventional known measures to prevent the lost circulation include a method of supplying lost circulation materials, such as raw cotton, sawdust and walnut shell, to the drilling mud, and a method of supplying cement slurry into a lost circulation layer and solidifying the cement slurry in the lost circulation layer. JPS59-118938 discloses adding hardening accelerator, such as aqueous solution of silicate of soda, to a cement type grout, wherein the hardening accelerator accelerates the hardening of the cement type grout.

SUMMARY OF INVENTION

The most reliable method for lost circulation that occurs during the drilling of a vertical hole is to supply cement slurry into the lost circulation layer. In that case, hardening accelerator may be added to the cement slurry, as disclosed in JPS59-118938. The hardening accelerator can accelerate the hardening of the cement slurry, efficiently gelate the cement slurry in the lost circulation layer and close the lost circulation layer. The cement slurry is supplied into the hole through a pipe that is called a cementing tool and that is provided in the hole. Thus, it is necessary to prevent the cement slurry from gelating or hardening in the cementing tool. Conventionally, in order to prevent the cement slurry from coming into contact with the hardening accelerator in the cementing tool. a spacer layer, such as water, is interposed between the cement slurry and the hardening accelerator.

However, the method of sequentially supplying the cement slurry, the spacer layer and the hardening accelerator complicates the process, leaving room for improvement. In addition, when the cement slurry is supplied into a hole for a purpose other than the drilling of a vertical hole, it is also desired that the cement slurry be prevented from coming into contact with the hardening accelerator in the cementing tool.

The present invention aims at providing a method of supplying cement slurry, the method being capable of preventing the cement slurry from coming into contact with the hardening accelerator in the cementing tool in a simple manner.

A method of supplying cement slurry according to the present invention comprising the steps of: adding a capsule to cement slurry, wherein the capsule is filled with hardening accelerator for the cement slurry; and supplying the cement slurry to a hole. The capsule is added to the cement slurry, and the cement slurry is supplied through a cementing tool that is provided in the hole. The capsule melts or collapses after a specific amount of time passes, wherein the specific amount of time is longer than a flow out time, which is a time for the capsule to flow out of the cementing tool after the capsule is added to the cement slurry.

When the cement slurry is transported in the cementing tool, the hardening accelerator is isolated from the surrounding cement slurry by the capsule. Therefore, according to the present invention, it is possible to provide a method of supplying cement slurry, the method being capable of preventing the cement slurry from coming into contact with the hardening accelerator in the cementing tool in a simple manner.

The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of an onshore rig to which the present invention can be applied;

FIGS. 2A and 2B are schematic views illustrating a method of preventing lost circulation according to an embodiment of the present invention;

FIG. 3 is a schematic view of a capsule;

FIGS. 4A and 4B are schematic views illustrating a method of preventing lost circulation according to a comparative example;

FIGS. 5A and 5B are schematic views of inner string cementing;

FIGS. 6A to 6E are schematic views of two-plug cementing;

FIGS. 7A and 7B are schematic views of squeeze cementing; and

FIGS. 8A and 8B are schematic views of plug-back cementing.

LIST OF REFERENCE NUMERALS

1 onshore rig

4 casing pipe

5 drill

9 mud pump

14 supply facility for cement slurry

17 slurry pump

19 capsule tank

20 capsule supply pipe

21 wall of the hole

22 annulus

31 hardening accelerator

32 coating material

51 bit

52 drill collar

53 drill pipe

54 injector

105, 501 cementing tool

206, 502, 603, 704 cement slurry

C, CH capsule

DESCRIPTION OF EMBODIMENTS First Embodiment

Some embodiments of the present invention will now be described with reference to the drawings. FIG. 1 shows a schematic view of onshore rig 1 according to an embodiment of the present invention. A platform called substructure 2 is provided on ground G, and scaffold 3 (also called derrick) is formed on the platform. Tubular casing pipe 4 is installed underground below substructure 2, and drill 5 (also called a drill string) that drills formation is suspended into casing pipe 4 by means of wire 6. Wire 6 is drawn out from draw works 7 that is provided on the top of substructure 2, and can raise and lower drill 5 via pulley 8 that is provided on the top of scaffold 3. Mud pump 9 that circulates drilling mud is provided on ground G or on the top of substructure 2. Mud tank 12 that stores the drilling mud is provided on ground G. Mud tank 12 is connected to mud pump 9 by line 13, and mud pump 9 is connected to drill 5 by line 10.

Drill 5 includes bit 51 that drills the formation, drill collar 52 that applies force to bit 51 and drill pipe 53 that is connected to drill collar 52. Drill pipe 53, as well as drill collar 52 and bit 51 that are connected to drill pipe 53, can be rotated around the central axis thereof by means of a rotational drive mechanism (not illustrated) that is provided above drill pipe 5. Drill collar 52 and drill pipe 53 are formed of steel pipes, each having an inner fluid passage that allows the drilling mud to pass through. The drilling mud that is supplied from mud pump 9 flows through line 10, drill pipe 53 and drill collar 52 to be injected toward the bottom of the hole from a nozzle that is provided in bit 51 that is located at the end of drill 5. The drilling mud that takes in cuttings, which are generated during the drilling of the formation using bit 51, is recovered while flowing upward through annulus 22 between drill 5 and wall 21 of the hole and is returned to mud tank 12 through line 11 on the ground. An apparatus (not illustrated) that removes the cuttings that are contained in the drilling mud is provided on line 11, so that mud pump 9 can circulate the drilling mud again free of the cuttings.

Furthermore, onshore rig 1 has facility for supplying cement slurry 14 in order to cope with lost circulation, described later. Facility for supplying cement slurry 14 includes cement slurry tank 15 that stores cement slurry, slurry pump 17 that is connected to cement slurry tank 15 via line 16 and that pumps the cement slurry into the hole, and line 18 that is connected to slurry pump 17 and that merges with line 10. Furthermore, facility for supplying cement slurry 14 includes capsule tank 19 that stores capsules that are filled with hardening accelerator for the cement slurry and capsule supply line 20 that is connected to capsule tank 19 and that merges with line 18. Accordingly, the capsules are added to the cement slurry downstream of the slurry pump. Valve 24 is provided on line 10 between mud pump 9 and the merging point with line 18, and valve 25 is provided on line 18. Valve 24 is open and valve 25 is closed during the drilling of the vertical hole. Valve 26 is provided on line 18 between slurry pump 17 and the merging point with capsule supply pipe 20, and valve 27 is provided on capsule supply line 20. Valves 26, 27 are closed during the drilling of the vertical hole.

Casing pipe 4 is used in order to protect wall 21 of the hole. In the illustrated example, outer casing pipe 4 a and inner casing pipe 4 b are provided. First, a vertical hole having a larger diameter than outer casing pipe 4 a is drilled, and outer casing pipe 4 a is then provided in the vertical hole. Next, annulus 22 a between outer casing pipe 4 a and wall 21 of the hole is filled with cement 23 a in order to fix outer casing pipe 4 a. This operation is a process of filling annulus 22 a with cement slurry (also called cement milk), which is a mixture of cement and water, and of solidifying the cement slurry. Then, drill 5 is lowered into outer casing pipe 4 a to drill a vertical hole having a larger diameter than inner casing pipe 4 b, and inner casing pipe 4 b is provided in the vertical hole. Then, annulus 22 b between inner casing pipe 4 b and wall 21 of the hole and between inner casing pipe 4 b and outer casing pipe 4 a is filled with cement 23 b in order to fix inner casing pipe 4 b. Thereafter, bit 51 is replaced and the drilling is further proceeded downward below inner casing pipe 4.

The drilling mud has the function of taking in cuttings that are generated during the drilling and of transporting the cuttings to the ground, the function of adjusting the pressure in the vertical hole, the function of protecting the wall of the hole in order to prevent the collapse of the formation, the function of cooling bit 51 and so on. The drilling mud is a suspension that is made by mixing water with clay, such as bentonite, but the drilling mud is not limited to this, and a wide variety of generally used drilling mud may be used.

Lost circulation may occur during the drilling of a vertical hole. The lost circulation is a phenomenon in which when a lost circulation layer that communicates with wall 21 of the hole exists in the formation, the drilling mud flows out of the hole into the lost circulation layer. Thus, the lost circulation is also called mud loss. FIG. 2A shows a sectional view of vertical hole 203 and lost circulation layer 202 that communicates with wall 21 of the hole. The interior of vertical hole 203 is filled with drilling mud 204, and vertical hole 203 is in contact with surrounding ground 205. When the mud loss occurs, a part of or the entire part of the drilling mud that is supplied to vertical hole 203 cannot be recovered. Accordingly, the occurrence of the lost circulation from wall 21 of the hole can be detected based on the flow rate of the drilling mud that is supplied and the flow rate of the drilling mud that is recovered. More specifically, if the flow rate of the drilling mud that is recovered is reduced below the flow rate expected from the flow rate of the drilling mud that is supplied, then it can be presumed that lost circulation has occurred. In addition, the position of bit 51 of drill 5 at the time when the flow rate of the drilling mud that is recovered has been reduced below the flow rate expected from the flow rate of the drilling mud that is supplied is highly likely to correspond to the depth at which the lost circulation has occurred.

Therefore, the possible location in vertical hole 203 where the lost circulation has occurred (how deep from the ground lost circulation layer 202 exists) can also be detected.

When the lost circulation is detected, drill 5 and mud pump 9 are stopped to halt the drilling, then drill 5 is raised out of the hole to the ground using wire 6, and bit 51 and drill collar 52 are detached. Then, as shown in FIG. 2B, injector 54 for injecting cement slurry is attached to the end of drill pipe 53, drill 5 is lowered again as cementing tool 105, so as to be installed in the hole. The injection port of injector 54 for injecting the cement slurry is provided near the bottom of the hole, which is the bottom that exists when the drilling is halted. In order to prevent cementing tool 105 from sticking in the hole, injector 54 preferably has about the same outer diameter as drill pipe 53. In addition, in order to prevent cementing tool 105 from sticking due to the hardening of the cement slurry, injector 54 may be spaced from the bottom of the hole with a certain distance therebetween. Injector 54 may be omitted.

Next, valve 24 on line 10 is closed, valves 25, 26 on line 18 are opened, and slurry pump 17 is activated in order to supply the cement slurry that is stored in tank 15 to line 10 via lines 16, 18. In order to prevent the backflow of the cement slurry into capsule supply line 20, valve 27 is kept closed during the operation of slurry pump 17. After the cement slurry is supplied to line 18, slurry pump 17 is stopped temporarily, valve 27 is opened, and valve 26 is closed. Thereafter, the capsules are supplied from capsule supply line 20 to line 18. The capsules may be supplied to line 18, for example, by their own weight. In order to smoothly supply the capsules to line 18, a liquid, such as cement slurry, may be put in capsule tank 19, and the capsules may be supplied to line 18 together with the liquid. Thereafter, valve 27 is closed, valve 26 is opened, and slurry pump 17 is activated to supply the cement slurry to line 18. By repeating these operations, the capsules that are filled with the hardening accelerator for the cement slurry can be added, via line 20, to the cement slurry that flows in line 18. As shown in FIG. 2B, cement slurry 206, to which capsules C are added, is supplied to the hole via cylindrical cementing tool 105 that is provided in the hole.

The hardening accelerator accelerates the hardening of the cement slurry. The hardening accelerator is preferably calcium chloride, silicate of soda (water glass) or mixture thereof. Calcium chloride is typically in granular or powdery form, but may be in the form of an aqueous solution. Silicate of soda is a thick aqueous solution of sodium silicate and exists in the form of malt syrup. The hardening accelerator may also be inorganic compound, such as chloride (NaCl, KCl), nitrous acid (Ca(NO₂)₂, KNO₂), nitrate (Ca(NO₃)₂, NaNO₃, KNOB), sulfate (CaSO₄, Na₂SO₄, K₂SO₄), thiocyanate (NaSCN), alkali (NaOH, KOH), carbonate (Li₂CO₃, Na₂CO₃, K₂CO₃), alumina-based compound (Al(OH)₃, Al₂O₃) and alkaline aluminate (NaAlO₂, Ca(AlO₂)₂), or inorganic compound, such as amine, gallium salt of organic acid and maleic anhydride. The hardening accelerator is not limited to these, and the hardening accelerator may be in liquid or in solid as long as it can fill the capsule.

As shown in FIG. 3, capsule C is formed of hardening accelerator 31, which is the contents of capsule C, and coating material 32 that coats hardening accelerator 31. Capsule C can be produced by a well-known process. The arrangement of capsule C is not limited. For example, coating material 32 may be formed of more than one layer. Capsule C, or more precisely, coating material 32 of capsule C gradually melts or collapses after capsule C is added to the cement slurry, and discharges hardening accelerator 31 that fills the interior to the cement slurry after a specific amount of time passes. The specific amount of time is set to be a little longer than the time for capsule C to flow out of cementing tool 105 after capsule C is added to the cement slurry (hereinafter, called flow out time). Thus, capsule C is prevented from melting inside cementing tool 105, and the cement slurry is prevented from hardening inside cementing tool 105. Accordingly, the cement that has hardened is prevented from remaining in cementing tool 105 or from clogging cementing tool 105. The flow out time may be calculated by dividing the total volume of the passage from a location where capsules C are added to the cement slurry (a location where capsule supply line 20 merges with line 18) to injector 54 by the flow rate of the cement slurry. The specific amount of time and the flow out time depend on the location of the lost circulation layer. When the lost circulation layer is located at a shallow position, the flow out time is short and the specific amount of time may be short. When the lost circulation layer is located at a deep position, the flow out time is long and the specific amount of time must be set long accordingly. In this case, it is preferable to prepare more than one kind of capsules C each having different melting time. Alternatively, the flow rate of the cement slurry that is supplied (the flow rate of slurry pump 17) may be controlled depending on the depth of the injection port or the depth of the lost circulation layer such that capsules C melt after they flow out of the injection port of injector 54. Specifically, when the injection port or the lost circulation layer is located at a shallow position, the flow rate of slurry pump 17 may be decreased and when the injection port or the lost circulation layer is located at a deep position, the flow rate of slurry pump 17 may be increased. As described above, the lost circulation is presumed, with a high probability, to have occurred near the position of bit 51 at a time when the flow rate of the drilling mud that is recovered has been reduced below the flow rate expected from the flow rate of the drilling mud that is supplied. However, the lost circulation may also occur at a shallower position, for example, in a lost circulation layer to which measures have been taken in the past against lost circulation. Accordingly, if the lost circulation does not stop after measures against the lost circulation have been taken targeting the vicinity of bit 51, the target may be changed to the lost circulation layer to which measures have been taken in the past against the lost circulation. In this case, the specific amount of time may be adjusted such that capsules C melt or collapse near the target lost circulation layer.

Capsule C may be formed of synthetic resin, natural polymer materials and so on. Capsule C has melting characteristics that depend on pressure, temperature or pH. In other words, the specific amount of time is a parameter of at least one selected from among pressure, temperature and pH. Since the hole is typically under a high temperature and a high pressure, capsule C preferably melts or collapses when a specific amount of time passes under a predetermined high temperature or a predetermined high pressure. Alternatively, capsule C may melt or collapse when a specific amount of time passes under both a predetermined high temperature and a predetermined high pressure. Since capsule C gradually falls in the hole, the pressure and the temperature vary with time, but the time until the melting occurs can be estimated through prior tests or simulations that take into consideration the variation of the pressure and the temperature. Since the cement slurry is strong alkali, it is also possible to use capsule C that melts or collapses under strong alkali when a specific amount of time passes.

FIG. 4A shows a method of supplying the hardening accelerator using a spacer layer instead of capsule C. In this comparative example, in order to prevent the cement slurry from hardening due to the hardening accelerator in cementing tool 105, after cement slurry 61 is supplied by cementing tool 105, spacer layer 62 that consists of water is supplied, and thereafter hardening accelerator 63 is supplied. However, as shown in FIG. 4B, cement slurry 61 that flows first into lost circulation layer 202 continues to proceed forward in lost circulation layer 202, and hardening accelerator 63 can never catch up with cement slurry 61. As a result, cement slurry 61, spacer layer 62 and hardening accelerator 63 need to be injected repeatedly in order to close lost circulation layer 202 with cement, but this is not desirable from the viewpoint of both time and cost.

In the present embodiment, capsules C and the cement slurry are simultaneously supplied in the hole. For this reason, immediately after capsules C melt, the hardening accelerator comes into contact with the cement slurry to cause the cement slurry to harden. In FIG. 2B, black capsules C conceptually indicate capsules C that have not yet melted, and white capsules C conceptually indicate capsules C that have melted. Since capsules C do not melt when they move inside cementing tool 105, and melt after they exit from cementing tool 105, capsules C can close the circulation lost layer in a short time and with a low cost and prevent the lost circulation from proceeding, while preventing the clogging of cementing tool 105.

Capsule C that is filled with the hardening accelerator preferably has about the same specific weight as the cement slurry. As a result, capsules C in the cement slurry can be transported at about the same velocity as the cement slurry. In addition, since capsules C can be uniformly distributed in the cement slurry, it is possible to cause a large amount of cement slurry to efficiently harden. If the specific weight of capsule C is much larger than the specific weight of the cement slurry, capsules C will flow out of cementing tool 105 earlier than the expected flow out time, and the melting of capsule C may not occur in the desired position. If the specific weight of capsule C is much smaller than the specific weight of the cement slurry, there is the possibility that capsules C do not flow out of cementing tool 105 due to buoyancy even after the flow out time passes. The specific weight of capsule C that is filled with the hardening accelerator is preferably 70% or more and 130% or less of the specific weight of the cement slurry, more preferably 90% or more and 110% or less of the specific weight of the cement slurry.

As described above, capsules C are added to the cement slurry downstream of slurry pump 17 that pumps the cement slurry into the hole. This is intended to prevent capsules C from being broken by slurry pump 17, but if there is no large concern about that, capsule C may be added to the cement slurry upstream of slurry pump 17, as shown by the broken line in FIG. 1. Specifically, instead of line 20, line 20 a that connects capsule tank 19 to line 16 is provided, and valve 27 a is provided on line 20 a. The cement slurry and the capsules may be alternately supplied by switching valve 26 and valve 27 a alternately, as in the above embodiment. However, for example, if the capsules can be supplied into line 16 by their own weight, then slurry pump 17 may be operated while keeping both valve 26 and valve 27 a open. Due to the agitating effect of slurry pump 17, capsules C are further uniformly distributed in the cement slurry.

The hole may be an inclined hole or a lateral hole. Lost circulation may also occur in an inclined hole or a lateral hole during the drilling, and measures may be taken to prevent the lost circulation in the same manner.

Second Embodiment

The present invention can also be applied to the cementing. Cement generates column hydrostatic pressure when it is in the form of fluid, but when the cement is solidified, loss of the column hydrostatic pressure occurs, which causes the possibility that fluid, such as gas, water and oil, cannot be prevented from gushing from the bottom of the hole. In addition, the cement that has hardened may generate fine cavities therein, called micro annuluses, that may become passages for fluid, such as gas, water and oil. Fluid, such as gas, water and oil, that flows into the hole may penetrate through the cement layer via the micro annuluses of the cement so as to gush out to the ground. In addition, cement slurry that is in the process of hardening easily allows fluid, such as gas, water and oil, to pass therethrough, and such phenomenon may cause the fluid to gush out to the ground while the cement slurry is in the process of hardening or may generate cavities in the cement after the cement slurry has hardened.

FIGS. 5A, 5B show the processes of inner string cementing to which the present invention is applied. As shown in FIG. 5A, in the inner string cementing, cement slurry 502, to which capsules C are added, is supplied from cementing tool 501 to casing pipe 4. Casing pipe 4 is provided with floating collar 503. Cement slurry 502 is prevented from flowing out to the side of cementing tool 501 by the end of cementing tool 501 abutting against floating collar 503. Cement slurry 502 is pumped into casing pipe 4, turns around the lower end of casing pipe 4 so as to fill annulus 505 between casing pipe 4 and wall 504 of the hole. Thereafter, as shown in FIG. 5B, cementing tool 501 is raised out of the hole. The cement that remains in casing pipe 4 is drilled by the bit and is removed in a later process. In the present embodiment, since capsules C melt after they flow out of cementing tool 501, cementing tool 501 is prevented from sticking due to the cement that has hardened. It should be noted that casing pipe 4 is an example of a hollow cylindrical body, and the hollow cylindrical body may also be, for example, a liner. As described above, the specific weight of capsule C that contains the hardening accelerator is preferably about the same as the specific weight of cement slurry 502.

FIGS. 6A to 6E show the processes of two-plug cementing to which the present invention is applied. In the two-plug cementing, the casing pipe itself is used as a passage to supply the cement slurry. After casing pipe 4 is provided, as shown in FIG. 6A, first plug 601 is installed, as shown in FIG. 6B. Next, preceding water 602 is supplied in order to prevent the cement slurry from mixing with the drilling mud and to wash the inside of casing pipe 4. Then, cement slurry 603 is supplied. Thereafter, as shown in FIG. 6C, second plug 604 is installed, and displacement water (mud) 605 is supplied in order to pump cement slurry 603. As shown in FIG. 6D, first plug 601 engages floating collar 606 that is provided in casing pipe 4 between both ends thereof and is broken by back pressure. As a result, cement slurry 603 flows downward in casing pipe 4, turns around the lower end of casing pipe 4 to fill annulus 608 between casing pipe 4 and wall 609 of the hole, as shown in FIGS. 6D and 6E. Thereafter, cement slurry 603 hardens. The cement that remains in casing pipe 4 is drilled by the bit and is removed in a later process. In the present embodiment, since a cementing tool is not used, the clogging of a cementing tool does not occur. However, if cement slurry 603 hardens in casing pipe 4, then cement slurry 603 may not reach every part of annulus 608. Using capsule C to control the timing of the hardening ensures that annulus 608 is filled with cement slurry 603.

FIGS. 7A, 7B show the processes of squeeze cementing to which the present invention is applied. The squeeze cementing is a process of remedying imperfect cementing. When a defect, such as a cavity or crack 702 occurs in cement 701 due to reasons, such as imperfect operation and aging, as shown in FIG. 7A, a process to remedy the defect may be required. Here, suppose that hole 705 or a crack is generated on the side of casing pipe 4 and that hole 705 or the crack communicates with a cavity or crack 702 on the backside of casing pipe 4. In this case, as shown in FIG. 7B, remedying pipe 703 (also called an injection-sub) is inserted into the hole, and cement slurry 704, to which capsules C are added, is pumped into the cavity or crack 702 on the backside of casing pipe 4 through the opening that is provided on the side of remedying pipe 703 and through hole 705. In this operation, packers 706 are preferably provided above and below the opening of remedying pipe 703 in order to supply cement slurry 704 to a predetermined depth region that includes hole 702. Thus, cement slurry 704 can be efficiently supplied to the backside of casing 4 through hole 702. Cement slurry 704 hardens and closes the cavity or crack 702. When the specific weight of capsule C is about the same as that of cement slurry 704, capsules C are uniformly distributed in cement slurry 704. When there is no hole 702 or crack that is open on casing 4 or when hole 702 or the crack is small, hole 702 for pumping the cement slurry may be made in casing 4. In the present embodiment, since capsules C melt after they flow out of remedying pipe 703, remedying pipe 703 is prevented from sticking due to the cement that has hardened.

FIGS. 8A, 8B show the processes of plug-back cementing to which the present invention is applied. The plug-back cementing is used for abandonment, regeneration and so on of a vertical hole. When a vertical hole is abandoned, a plug called cement plug 801 is formed at the bottom of the hole in order to prevent gas or water from gushing out, as shown in FIG. 8A. In case of a deep hole, more than one plug is sequentially formed, and as a result, plugs may also be provided in the middle or upper parts of the hole. When a vertical hole is regenerated, new vertical hole 803 is branched obliquely from old vertical hole 802 at a part between both ends thereof, as shown in FIG. 8B. This operation is called side track because new vertical hole 803 is drilled from the side of old vertical hole 802. In addition, although not illustrated, when the bit sticks during the drilling of the vertical hole, the drill collar or the drill pipe immediately above the bit may be cut, and a new bit may be attached in order to continue the drilling. This is also called side track, as in the case of regenerating a vertical hole. In both cases, plug-back cementing is carried out in order to prevent gas and water from gushing out from the bottom of old vertical hole 802. In the present embodiment, since cement slurry, to which capsules C are added, is supplied, cement plug 801 can be properly formed on the bottom of the hole by capsules C melting near the bottom of the hole.

In the present embodiment, capsule CH having a larger specific weight than the cement slurry may be used. The specific weight of capsule CH may be adjusted depending on the material of coating material 32. Alternatively, a material having a large specific weight may be added to hardening accelerator 31. As shown in FIGS. 8A and 8B, capsule CH having a larger specific weight than the cement slurry concentrates at bottom 803 of the hole due to the difference of the specific weight. For this reason, the hardening accelerator can be distributed at bottom 803 of the hole with a high density, and the cement slurry at bottom 803 of the hole can harden more quickly and more properly. Thus, it is possible to quickly prevent gas and water from gushing out from bottom 803 of the hole and to maintain the integrity of old vertical hole 802 in the abandonment of a vertical hole or in the side track. In addition, by designing capsules CH such that they melt after they reach bottom 803 of the hole, the cement slurry at bottom 803 of the hole can harden more properly.

Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made without departing from the spirit or scope of the appended claims. 

1. A method of supplying cement slurry comprising the steps of: adding a capsule to cement slurry, wherein the capsule is filled with hardening accelerator for the cement slurry; and supplying the cement slurry to a hole, wherein the capsule is added to the cement slurry, and the cement slurry is supplied through a cementing tool that is provided in the hole, wherein the capsule melts or collapses after a specific amount of time passes, wherein the specific amount of time is longer than a flow out time, which is a time for the capsule to flow out of the cementing tool after the capsule is added to the cement slurry.
 2. The method of supplying cement slurry according to claim 1, wherein the capsule is added to the cement slurry downstream of a slurry pump that pumps the cement slurry into the hole.
 3. The method of supplying cement slurry according to claim 1, wherein the capsule is added to the cement slurry upstream of a slurry pump that pumps the cement slurry into the hole.
 4. The method of supplying cement slurry according to claim 1, wherein the hardening accelerator is calcium chloride, silicate of soda or mixture thereof
 5. The method of supplying cement slurry according to claim 1, wherein the capsule has melting characteristics that depend on at least one selected from among pressure, temperature and pH.
 6. A method of drilling a hole comprising the steps of: drilling a hole using a bit that is provided at an end of a drill while supplying drilling mud to the hole through an inner fluid passage of the drill and recovering the drilling mud from an annulus between a wall of the hole and the drill; stopping the drilling using the drill when occurrence of lost circulation from the wall of the hole is detected based on a flow rate of the drilling mud that is supplied and a flow rate of the drilling mud that is recovered; and supplying cement slurry to the hole after the drilling is stopped, wherein a capsule is added to the cement slurry by the method of supplying cement slurry according to claim
 1. 7. The method of drilling a hole according to claim 6, wherein an injection port for the cement slurry of the cementing tool is provided near a bottom of the hole, the bottom existing when the drilling is stopped, and the flow rate of the cement slurry that is supplied is adjusted depending on depth of the injection port such that the capsule melts or collapses after the capsule flows out of the injection port.
 8. The method of drilling a hole according to claim 7, wherein the specific amount of time is adjusted such that the capsule melts or collapses near a location where the lost circulation occurs.
 9. The method of drilling a hole according to claim 6, wherein the drill is raised out of the hole after the drilling is stopped, and the drill is provided in the hole again as the cementing tool after the bit is detached.
 10. The method of drilling a hole according to claim 6, wherein the capsule that is filled with the hardening accelerator has about a same specific weight as the cement slurry.
 11. A method of cementing comprising the steps of: supplying cement slurry to a hole through a cementing tool that is provided in a hollow cylindrical body in the hole, wherein the cement slurry is supplied by the method of supplying cement slurry according to claim 1, wherein the cement slurry turns around a lower end of the hollow cylindrical body into an annulus between the hollow cylindrical body and a wall of the hole, flows upward through the annulus to fill the annulus, thereby the hollow cylindrical body is fixed to the wall of the hole.
 12. A method of cementing comprising the steps of: supplying cement slurry to a hole through a cementing tool that is provided in the hole, wherein the cement slurry is supplied by the method of supplying cement slurry according to claim 1, wherein, a cement plug is formed at a bottom of the hole by the cement slurry.
 13. The method of cementing according to claim 12, wherein the capsule that is filled with the hardening accelerator has a larger specific weight than the cement slurry.
 14. A method of cementing comprising the steps of: supplying cement slurry to a hole through a cementing tool that is provided in a hollow cylindrical body in the hole, wherein the cement slurry is supplied by the method of supplying cement slurry according to claim 1, wherein a defect of cement between the hollow cylindrical body and a wall of the hole communicates with interior of the hollow cylindrical body via a hole that is open on a wall of the hollow cylindrical body, and the cement slurry is supplied to a predetermined depth region that includes the hole of the hollow cylindrical body, and the cement slurry fill the defect via the hole.
 15. A capsule that is filled with hardening accelerator for cement slurry, wherein the capsule is added to the cement slurry, and the cement to which the capsule is added, is supplied to a hole through a cementing tool that is provided in the hole, and the capsule melts or collapses after a specific amount of time passes, wherein the specific amount a time is longer than a flow out time, which is a time for the capsule to flow out of the cementing tool after the capsule is added to the cement slurry.
 16. The capsule according to claim 15, wherein the hardening accelerator is calcium chloride, silicate of soda or mixture thereof.
 17. The capsule according to claim 15, wherein the capsule has melting characteristics that depend on at least one selected from among pressure, temperature and pH.
 18. The capsule according to claim 15, wherein the capsule that is filled with the hardening accelerator has about a same specific weight as the cement slurry.
 19. The capsule according to claim 15, wherein the capsule that is filled with the hardening accelerator has a larger specific weight than the cement slurry. 